With the development of fiber optic technologies, optical fiber gas sensors are playing an increasingly important role in environmental, safety and industrial process monitoring as well as national security applications. They have predominant features such as light weight, small size, remote detection capability, safety in hazardous environments, immunity to electromagnetic interference. Optical fiber sensor and system based on direct absorption spectrometry (DAS) is well-known and widely used in the identification of chemical species including gases. According to Beer-Lambert law, when a light with specific wavelength passes through a target gas, a portion of the light energy is absorbed by the target gas, so that the transmission light power is reduced, through which the concentration of the target gas is analyzed. The method is simple and effective, but its sensitivity is relatively low and limited by the light absorption length and a variety of noises.
Another commonly used method is tunable diode laser absorption spectrometry (TDLAS), which measures the concentration of the target gas through the change of the absorption strength when the wavelength of a tunable laser sweeps over a gas absorption line. Combined with amplitude modulation (AM) and/or wavelength modulation (WM) techniques, the method can effectively reduce the impact of noise of laser and other background noise and thus a higher sensitivity in gas measurement can be achieved. However, this method is still limited by the absorption length, a variety of methods of increasing the absorption length make the system become complex and large where high precision free-space optical components and alignments are required.
Fiber optic gas sensors with an open-path absorption cell comprising a pair of aligned miniature gradient-index (GRIN) lenses with fiber pigtails just performs a purely passive role in transferring light to and from the absorption cell but plays no part in gas sensing. Conversely, hollow-core optical fiber allows the confinement of light and gas simultaneously in its hollow-core, and this provides an excellent platform for strong light-gas interaction inside the fiber core over a long distance by which the fiber plays a more direct role in the sensing process. The fundamental mode optical field propagates in the optical fiber interacts with the gas, the absorption spectrum thereof or the attenuation of the laser power is proportional to the concentration of the gas, such that the concentration of the gas may be determined. The hollow-core optical fiber serves as an absorption cell can easily achieve a longer absorption length, thereby improving the measurement sensitivity; the optical fiber can be bent to form a fiber coil with a small diameter with negligible loss, to achieve a smaller sensor head. Thus, recently it more and more intends to use hollow-core optical fiber to measure the concentration of the gas. However, the current hollow-core optical fiber supports some high-order modes besides fundamental mode, the noise interference between the modes affects the measurement sensitivity.
Another gas measurement method based on spectral absorption is photothermal spectrometry/photoacoustic spectrometry (PTS/PAS). Distinguished from the above direct absorption measurement method, the spectrometry/photoacoustic spectrometry indirectly measures the change in temperature or in acoustic wave generated after the gas absorbs light, so as to obtain gas concentration information. Compared with the direct absorption method, a signal generated by this method is directly proportional to the amount of the absorption and is not affected by background light noise. Using high power lasers and high sensitivity acoustic or thermal detector can achieve extremely low gas concentration detection limit (ppb or even ppt). However, using this method to measure requires the use of an electrical detector, and can only achieve a single point measurement, which may not meet a variety of requirements of multi-point and remote sensing.